Compound semiconductor epitaxial structure and method for fabricating the same

A method for fabricating a compound semiconductor epitaxial structure, comprising steps of:

• forming an undoped silicon germanium layer having a linearly-increased concentration gradient of germanium on a substrate;
• forming a first compound epitaxial layer on the undoped silicon germanium layer;
• performing a continuous epitaxial deposition process to form a second compound epitaxial layer on the first compound epitaxial layer, so that the second compound epitaxial layer has a linearly-decreased concentration gradient of metal;
• and
• forming a semiconductor material layer on the second compound epitaxial layer.